E-mail traffic is one of the rapidly growing applications in mobile/radio communication networks. E-mails are downloaded to laptops or mobile terminals or other radio/wireless communication devices through mobile communication networks.
Because available bandwidth is typically limited in mobile communication networks, handling of large e-mails and/or e-mails with attachments (for example over 20 kB) often leads to poor user experience due to long periods of time needed for downloading and sending e-mails. Furthermore, the user is often billed according to the amount of transferred data. Therefore it would be beneficial for the user to be able to minimise the amount of data that is transferred.
For these reasons methods of reducing the amount of transferred data have been developed.
In the following, terminology that is used for describing such methods is briefly discussed. This terminology will be adhered to in the rest of this document.                Lossless data processing: it is possible to revert to the original data on the basis of the processed data; no information is lost.        Lossy data processing: it is not possible to revert to the original data as a whole on the basis of the processed data; at least part of the information is lost.        Compression: lossless method, in which information is stored in fewer bits than the length of the original information.        Optimisation: lossy method, in which some parts of the original information are removed for reducing the size of the information.        Downgrading: reducing quality of the information for reducing its size.        Manipulation: removing or replacing parts of the original information for reducing its size.        Hashing: transformation of a string of characters into a shorter string that substantially uniquely identifies the original string.        Hash (value): value or key substantially uniquely identifying a longer string of characters. (Depending on the hash function different character strings may obtain the same hash value, but in most hash applications such collision is highly improbable and does not cause any practical problems.)        Differentiation: a change listing; calculation of differences between first and second versions of a file so that the second file can be obtained by combining the first file and the difference and the first file can be obtained by extracting the difference from the second file.        Low bandwidth network: a network in which there is one or more slow speed links along the path between a terminal and a server. Herein slow means that the link causes (significant) delays to the message transfer, that is, the message content size divided by the link speed is so large that the user perceives link slowness. Thus this definition does not specify the link speed in quantitative terms; instead the network bandwidth may be arbitrary high in case the content size is comparatively high.        
Currently optimisation is used only for downloading e-mails, whereas in sending of e-mails only compression is currently used. Compression may be, and often is, used also for downloading e-mails in addition to (or instead of) optimisation.
FIGS. 1A-1C show diagrams illustrating different e-mail setups in mobile networks. All setups are shown in a system comprising a mobile terminal that is connected to Internet/operator's GPRS (General Packet Radio Service) intranet via a mobile/radio network.
FIG. 1A shows a basic setup, wherein the mobile terminal comprises an e-mail client 101 and Internet/operator's GPRS network comprises an e-mail server 102. The e-mail client 101 in the mobile terminal and the e-mail server 102 in the Internet/operator's GPRS intranet may communicate by using POP (Post Office Protocol), IMAP (Internet Message Access Protocol) or SMTP (Simple Mail Transfer Protocol) protocols, which are commonly known e-mail protocols. In this basic setup, there are no measures for accelerating e-mail transfer over the mobile/radio network.
FIG. 1B shows an optimised setup. Herein the Internet/operator's GPRS intranet comprises an e-mail server 112 and an (optimising) e-mail proxy 113 that is connected to the e-mail server 112 and communicates with the e-mail server by using POP, IMAP or SMTP protocol.
Two alternative setups are shown for the mobile terminal. In the first one the mobile terminal comprises an e-mail client 110 and an e-mail proxy client 111 that is connected to the e-mail server 112 and communicates with the e-mail server by using POP, IMAP or SMTP protocol. The e-mail proxy 113 and the e-mail proxy client 111 communicate with each other over the mobile/radio network by using a proprietary e-mail proxy protocol, which protocol is configured to optimise the transfer of e-mails to the e-mail proxy client. The e-mail proxy 113 compresses e-mail messages and the e-mail proxy client 111 decompresses them in the mobile terminal and provides them onwards to the e-mail client 110. In addition to this, the e-mail proxy 113 may optimise the emails to be downloaded by removing attachments from the e-mails or replacing them with HTTP URIs (HyperTexts Transfer Protocol, Uniform Resource Identifier) for allowing the user of the mobile terminal to download the attachments later.
E-mails are sent from the mobile terminal through the e-mail proxy client. The e-mail proxy client compresses the messages and sends them (in compressed form) to the e-mail proxy, which decompresses them and sends them onwards to the email server in decompressed form.
In the second mobile terminal setup the mobile terminal comprises only an e-mail client 114. Herein, the e-mail client 114 communicates directly with the e-mail proxy 113 by using POP, IMAP or SMTP protocol.
FIG. 1C shows a web interface setup. Herein, the setup of the Internet/operator's GPRS intranet corresponds to the setup of FIG. 1B. The mobile terminal comprises a www browser, which communicates with the e-mail proxy 113 in Internet/operator's GPRS intranet over a HTTP (HyperText Transfer Protocol) or HTTPS (Secure HTTP) connection. The user of the mobile terminal reads e-mails through a web interface. The user can decide which e-mails and/or which attachments are downloaded. That is, the user can decide to read only interesting messages and to download only required attachments. Thus, there is no need to download everything.
The disadvantage of the currently used solutions is that sent e-mails are only compressed and not optimised. Further, the web interface that is needed for optimising e-mail downloads is not optimal. For example, the e-mail messages are not saved for offline use and a considerable amount of additional data needs to be sent for establishing the web interface (www pages, images etc.).